Such apparatus is known and has many fields of use for particle size analysis of dispersed substances or drop distributions. In that case use of made of the fact that a particle irradiated by monochromatic light deflects this light to different degrees in dependence on its size, wherein small particles more strongly deflect the light than large particles.
The monochromatic light, usually produced by a laser, is accordingly diffracted by particles which are disposed in a measuring zone. The diffracted light has an angular distribution of intensity which corresponds to the size distribution of the illuminated particles. This angular distribution is converted into a positional distribution by an imaging device in the form of a convergent lens with a focal length f, which intercepts the beam diffracted at the particles, at a spacing therefrom corresponding to the focal length of the convergent lens. This positional distribution is picked up by a photo-detector arranged in the focal plans of this convergent lens and determined by measuring technology by a downstream electronic system. The distribution of the particle sizes is ascertained from the determined intensity distribution by a calculation algorithm (for example, Fraunhofer's Diffraction, Mie Theory). The particles can flow through the measuring zone dry or dispersed as an aerosol in a free jet or they can be dispersed in a liquid which is conducted through a measuring cell arranged in the measuring zone.
Normally, the measuring zone is arranged in the parallel beam path in front of the convergent lens. Then, however, due to the limited area of the conventionally employed photo-detectors only a specific range of particle sizes can be measured in the case of a convergent lens having a predetermined focal length, since, for example, for the measurement of larger particles a convergent lens with a larger focal length is necessary in order to achieve, on the photo detector, an acceptable image of the diffraction pattern which is formed by the beam deflected relatively weakly at the large particles. The large focal length of the lens leads to a correspondingly large housing for the measuring device.
Conversely, for the measurement of very small particle sizes, a lens with a correspondingly small focal length is required, which in addition should still have a largest possible aperture in order to still catch the beam very strongly deflected by the smallest particles. Such lenses having a short focal length inevitably produce, if they are to be affordable, large imaging errors. Accordingly, for the measurement of very small particle sizes it has been proposed (see European patent 0 207 170 B1) to arrange the measuring zone in the convergent beam path between convergent lens and photo-detector. This arrangement is certainly mathematically equivalent, except for a phase factor, to the arrangement in the parallel beam path, but the problem arises that now the distance between the particles and the photo-detector decides the measuring range. This distance cannot be accurately defined, as the guiding of the particles through the measuring zone must be designed for the largest particles to be measured, otherwise an undesired lack of definition in the position inevitably arises in the case of smaller particles flowing through the measuring zone.
In the case of samples with unknown particle size distribution an imaging device with a longer focal length and a correspondingly large measuring range must therefore be used for the determination of the particle size distribution in order to ensure that the largest particles contained in the sample are also detected in the determination. However, in the range of smaller particle sizes the long focal length brings with it a reduced resolution and thus a lower accuracy.
An object of the invention is to provide a device by which unknown distributions of particle sizes can be measured with high resolution in a short time.